Electrical conductor having transposed conducting members



May 12, 1959 w. H. DOHERTY 2,886,628

ELECTRICAL CONDUCTOR HAVING TRANSPOSED CONDUCTING MEMBERS Filed June 30,1955 2 Sheets-Sheet 1 IN VEN 7-0 W. H. OOHERTV A T TORNEV May 12,1959 w.H. DOHE-RTY 2,886,628

ELECTRICAL CONDUCTOR HAVING TRANSPOSED CONDUCTING MEMBERS Filed June 30,1955 2 Sheets-Sheet 2 IN l/E/V TOR W H. DOHER T Y A TTORNES ELETRICALCONDUCTQR HAVING TRANS- PUSED CONDUCTING MEMBERS William H. Doherty,Summit, NJ, assignor to Bell Telephone Laboratories, Incorporated, NewYork, N.Y., a corporation of New York Application June 30, 1955, SerialNo. 519,206

3 Claims. (Cl. 174-34) This invention relates to electrical conductorsand, more particularly, to electrical conductors each comprising aplurality of transposed conducting elements, and to methods of makingthem.

The term transposed conducting elements is used to refer to a pluralityof conducting elements whose positions are shifted relative to eachother and to an axis of the conductor. The shifting may be continuousalong the length of the conductor, or it may occur at discreteintervals. Also, the transposition may take the form of frequentshifting of large numbers of conducting strands, as in the case ofLitzendraht wire, or it may be an orderly change of position of a fewconductors such as, for example, the type shown and described in mycopending United States patent application Serial No. 366,- 510, filedJuly 7, 1953, now Patent 2,812,502 issued on November 5, 1957.

It is an object of this invention to simplify the construction ofelectrical conductors having transpositions.

It is another object of this invention to provide an electricalconductor the alternating-current resistance of which is less dependentupon frequency changes than that of conventional conductors.

In the transmission of electromagnetic waves, the current distribution,which is substantially uniform throughout the cross-sectional area ofthe conductor at very low frequencies, becomes nonuniform as frequencyis increased. Consider, for example, the case of a solid conductor towhich are applied waves of increasing frequency. At direct current andat very low alternating current frequencies, the current issubstantially uniformly 4 distributed throughout the cross-sectionalarea of the conductor, and the resistance of the conductor, and hencethe conductor loss, is at a minimum. As the frequency is increased, thecurrent density becomes a maximum at that surface of the conductor whichis exposed to the main field of the waves, which, in the presentexample, is the outer surface, and decreases as distance from the fieldincreases, i.e., towards the center of the conductor. The rate at whichthe current density decreases is dependent upon the frequency and thematerial of the conductor, and, for most conducting materials, at highfrequencies the current density at the center of the conductor becomesnegligible while the current density at the conductor surface is amaximum. This phenomenon is commonly known as skin effect, and a skindepth is defined as the distance measured inwardly from the StatesPatent surface of the conductor in which the current in the ICCstantially reduced if the conductor is formed of a number of conductingelements or members connected in parallel and transposed often so thateach conductor receives its share of exposure to the main field. Thisamounts to forcing the current to distribute itself over the entirecross-sectional area of the composite of individual con ductingelements, thereby increasing the total current carrying area. It followsthen that the alternating-current resistance is decreased substantially,and the frequency dependency of the alternating-current resistance islikewise decreased.

One Well-known example of a composite conductor which utilizes theforegoing principles is Litzendraht wire, which, while effective atlower frequencies, suffers from many disadvantages at the higherfrequencies. First, each individual strand of wire must be insulatedfrom all of the others, requiring great care in fabrication, especiallyfor operation at the higher frequencies. Second, for effectiveness atthe higher frequencies, the diameters of the individual strands of wiremust be made so small that of necessity there are great sacrifices instrength and ruggedness. Third, with the large number of individualstrands involved, proper transposition is exceedingly difficult toachieve, especially when use at high frequencies is contemplated, whichrequires numerous transpositions at very short intervals.

In accordance with the present invention, an electrical conductor havingtransposed conducting elements is provided which is a great improvementin many respects over other known transposed conductors. In anillustrative embodiment of the invention, a thin flat elongated sheet ofdielectric material is interposed between two thin fiat sheets ofconducting material, forming a fiat, elongated sandwich. The sandwich isthen rolled into the shape of an elongated hollow cylinder, with the twoedges of the sandwich in close proximity, forming a longitudinal seam orslot. At intervals along its length the cylinder is turned inside outand reformed into cylindrical shape, with what was the inner conductorbecoming the outer conductor, and what was the outer conductor becomingthe inner.

The invention will be more readily understood by referring to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

Fig. l is a perspective view of the arrangement of conductors andinsulators into a sandwich out of which the several preferredembodiments of the invention are made;

Fig. 2 is a plan view of an intermediate step in the formation of apreferred embodiment of the invention;

Fig. 3 is a plan view of a preferred embodiment of the invention showinga single composite conductor with transposed conducting elements;

Fig. 4 is a perspective view of the conductor of Fig. 3;

Fig. 5 is a perspective view of a coaxial cable embodying the principlesof the invention;

Fig. 6 is a cross-sectional view of a modified form of the cable of Fig.5;

Fig. 7 is a side view of a coaxial cable embodying the principles of thepresent invention;

Fig. 8 is a side view of still another coaxial cable embodying theprinciples of the present invention; and

Fig. 9 is a cross-sectional view of the composite conductor of thepresent invention as used in a twin wire transmission line.

Turning now to Fig. 1, there is shown a composite conductor 11comprising two thin, flat elongated sheets 12 and 13 of conductingmaterial, such as copper, separated by a thin, elongated sheet 14 ofdielectric or insulating material. Sheet 14 may be of any one of anumber of suitable materials, such as polystyrene, which have relativelylow dielectric constants. Conducting t 3 sheets 12 and 13 are preferablybonded to dielectric sheet 14 to. form a permanent composite conductor.The bonding may be accomplished by any one of a number of processes wellknown in the art, such as vapor deposition or plating the conductingmaterial onto the dielectric, or by use of suitable adhesive material.The thickness of conductors l2 and i3, and of dielectric 14 arepreferably of the order of one skin depth at the highest frequency ofoperation of the conductor, although some variation in thickness ispermissible.

In Figs. 3 and 4 there is shown a transposed conductor 15 which has beenformed from the composite conductor 11 of Fig. 1 in the followingmanner. Conductor 11 is rolled into the shape of a hollow cylindricalconductor with the two edges in proximity to each other, forming alongitudinal slot or seam. It is not necessary that the edges abut eachother. At intervals along the length of cylindricalv conductor 15, theslot is spread as best seen in-Fig. 2 and the cylinder is inverted,i.e., turned inside out, as best seen in Figs. 3 and 4 at points a andb. It should be noted that conductor 15 is widest at points a and b asillustrated in Figs. 3 and 4. The width at these points on the conductorwill be determined by the material used and the method of forming theinversion, and need not be substantially greater than the outsidediameter of the remainder of conductor 15. The slot which, in Fig. 2, isshown at the top of cylindrical conductor 15 in the first portion of itslength, is situated at the bottom of the cylinder after thetransposition at point a, and again at the top after the transpositionat point b. This 180-degree shift of the seam occurs at eachtransposition throughout the length of the conductor 15. It can readilybe seen in Figs. 3 and 4 that, after the transposition at point a, innerconductor 12 has become the outer conductor, and outer conductor 13 hasbecome the inner conductor, thus effecting a complete transposition. Ifthe main field along the conductor is at the outer surface, and if theintervals between transpositions remain the same throughout theconductor length, it is obvious that conductors 12 and 13 are made toshare equally exposure to the main field.

The effectiveness of transposition of conducting elements in a conductordepends to a considerable extent upon a judicious selection of thetransposition interval, that is, the distance, 1 between the points aand b in Figs. 3 and 4. It is desirable to transpose often enough toinsure a substantial reduction in losses, yet, on the other hand, in theinterests of ease and low cost of fabrication, it is desirable that theconducting elements be transposed no more frequently than is necessary.As long as the currents carried by the conducting elements areapproximately equal, losses will be minimized, hence it is necessary totranspose only often enough to maintain this condition of approximateequality of currents in the conducting elements. In the drawings of thepresent invention, the particular embodiments shown are neither to scalenor proportion, inasmuch as the transposition interval may be quite longas compared to that portion of the length of the conductor over whichthe transposition is accomplished.

In Fig. there is shown a coaxial cable 19 having a dielectric core 21, acomposite inner conductor 22, an outer conductor 23 spaced from theinner conductor 22, and a dielectric filling the space between the innerand outer conductors. Dielectric may be a solid dielectric or it may beair, in which case conductors 22.

and 23 are spaced from each other by dielectric spacers, not shown, ofthe type well known in the art. In the cable of Pig. 5, inner conductor22 is the type of composite conductor shown in Figs. 3 and 4, havingperiodic transpositions of its conducting elements along its length. Itis necessary to pierce conductor 22 in the manner shown at point a inFig. 5 to permit passage of core 21. Piercing conductor 22 in thismanner has a negligible effect on the overall efliciency of thearrangement.

In Fig. 6 there is shown a coaxial cable 25 having a core 26 of metal, acomposite inner conductor 28 separated from the core 26 by a thin layerof insulating material 27, and an outer conductor 29 separated frominner conductor 28 by dielectric material 31. As was the case in thecable of Fig. 5, dielectric 31 may be of solid dielectric material, orit may be air, in which case dielectric spacers are used to maintainconductors 23 and 2% in spaced relationship. Inner conductor 23 is ofthe type shown in Figs. 3 and 4, having periodic transpositions of itsconducting elements along its length. Core 26 is preferably of a metalhaving good conducting properties, in which case it contributes to theconduction of the inner conductor, and can also be used to carry powercurrents. Where mere mechanical strength alone is desired, core 26 canbe made of any suitable material. As was the case in the cable of Fig.5, the inner conductor 28 must be pierced at the transposition points topermit passage or" the core 26. The insulating layer 27 on core26efiectively prevents any metal to metal contact at the transpositionpoints.

In Fig. 7 there is shown a coaxial cable 32 having a solid innerconductor 33 and a composite outer conductor 34 separated from innerconductor 33 by a dielectric 35. Outer conductor 34 is similar tocomposite conductor 15 of Figs. 3 and 4 except that it is pierced at thetransposition points in a manner similar to the inner conductors of thecables of Figs. 5 and 6 to permit passage of the solid inner conductorand the dielectric. As was the case with the conductors of Figs. 5 and6, dielectric 35 may be of any suitable material, or it may be air, inwhich case dielectric spacers are used, and it is only necessary topierce conductor 34 sufficiently to permit passage of conductor 33.

Fig. 8 shows a coaxial cable having a core 41, an inner compositeconductor 37 and an outer composite conductor 38 separated therefrom bya dielectric 39. Dielectric 39 may be any suitable material or air, aswas the case in Figs. 5, 6 and 7. In the cable of Fig. 8, both conductor3"] and conductor 38 are similar to composite conductor 15 of Figs. 3and 4. Obviously, conductor 38 must be pierced at the transpositionpoints. Core 41 may be of dielectric material or metal, depending uponthe uses to which the'cable is to be put. If core 41 is of metal, a thincoating of insulation, not s town, is desirable. It is necessary thatconductor 37 be pierced at the transposition points to permit passage ofcore 41.

Figs. 5, 6, 7, and 8 all have dealt with the adaptation of the compositeconductor of Figs. 3 and 4 to coaxial cable However, the conductor ofFigs. 3 and 4 is readily adaptable to practically any current carryingconductor arrangement. For example, there is shown in Fig. 9 a parallelpair transmission line comprising composite conductors 42 and 43 mountedon cores 44 and 45 respectively. Conductors 42 and 43 are like theconductor of Figs. 3 and 4, but are pierced at the transposition pointsto permit passage of cores 44 and 45. Cores 44 and 45 may be either ofmetal or dielectric material like the core 41 of Fig. 8.

In all of the embodiments of the invention herein shown, the use ofinsulating material surrounding the cables, or of metallic sheathing ispermissible. There has been no showing of any insulator or sheathinginasmuch as such practices are well known in the art and have,therefore, been omitted for the sake of clarity.

It is to be understood that the above-described arrangements are merelyillustrative of the application of the principles of the invention, andapplicant does not intend to limit his invention to the particularembodiments herein shown. Numerous other embodiments may be devised bythose skilled in the art without departing from the spirit and scope ofthe invention.

What is claimed is:

l. A low loss electrical conductor of cylindrical form comprisinglaminations of conducting material separated by a sheet of insulatingmaterial, alternate sections of the conductor being formed by thecomposite laminations being bent in opposite directions to formcylinders, one of said laminations being on the inside in one of saidsections and the other of said laminations being on the inside in anadjacent section.

2. A low loss electrical conductor having a plurality of sectionscomprising continuous laminations of conduction material throughout saidsections, insulating material separating said laminations of conductionmaterial, said laminations being formed into substantially cylindricalportions throughout each of said sections with one lamination beingsurrounded by another lamination in one of said sections and said otherlamination being surrounded by said one lamination in an adjacent one ofsaid sections.

3. A low loss electrical conductor comprising laminated cylindricalsections of conducting material, said conducting material separated byinsulating material, the conductor being slotted through the laminationsand insulation for the entire length thereof, the slot of adjacentsections being at diametrically opposite points, and one of saidlaminations being on the inside in one of said sections and the other ofsaid laminations being on the inside in adjacent sections.

References Cited in the file of this patent UNITED STATES PATENTS2,052,317 Schelkunoif Aug. 25, 1936

